I don't know if this topic has been addressed before or not. A few that come to mind include:

Phil Collins (Genesis)
George Harrison (The Beatles)
Don Henley (The Eagles)
Michael Jackson (The Jackson 5)
John Lennon (The Beatles)
Paul McCartney (The Beatles)
Donny Osmond (The Osmonds)
Lionel Richie (The Commodores)
Linda Ronstadt (The Stone Ponies)
Diana Ross (The Supremes)
Ringo Starr (The Beatles)

And you forgot one:Frankie Valli of "The Four Seasons".

You mentioned Phill Collins (Genesis), but you forgot Peter Gabriel.

Here are some of mine. A lot of my picks never actually left their respective grups.

Eric Clapton (The Yardbirds, Cream, Blind Faith)
Chris Cornell (Soundgarden)
David Crosby (The Byrds, Crosby, Stills, & Nash)
John Fogerty (Creedence Clearwater Revival)
Glenn Frey (Eagles)
John Frusciante (Red Hot Chili Peppers)
Peter Gabriel (Genesis)
David Gilmour (Pink Floyd)
Lou Gramm (Foreigner)
Sammy Hagar (Montrose)
Taylor Hawkins (Foo Fighters)
Lauryn Hill (Fugees)
Wyclef Jean (Fugees)
Janis Joplin (Big Brother and the Holding Company)
Mark Knopfler (Dire Straits)
Van Morrison (Them)
Stevie Nicks (Buckingham/Nicks, Fleetwood Mac)
Steve Perry (Journey)
Tom Petty (Tom Petty and the Heartbreakers)
Robert Plant (The Yardbirds, Led Zeppelin)
David Lee Roth (Van Halen)
Paul Simon (Simon & Garfunkel)
Sting (The Police)
Gwen Stefani (No Doubt)
Rod Stewart (Faces)
Pete Townshend (The Who)
Tina Turner (Ike & Tina Turner)
Eddie Vedder (Pearl Jam)
Roger Waters (Pink Floyd)
Brian Wilson (The Beach Boys)
Dennis Wilson (The Beach Boys)
Neil Young (Buffalo Springfield)

Robert Palmer (Vinegar Joe)
Ben E. King (The Drifters)

There's even a site with performers who were in bands, but went solo....

http://www.amiright.com/names/better-off-solo/

Thanks to everyone who has contributed! I learned a lot; I had no idea there
were so many!

Ozzy Osbourne (Black Sabbath)
Ronnie James Dio (Ronnie & the Prophets, Elf, Rainbow, Black Sabbath)
Rob Halford (Judas Priest, Fight, still has self-titled band Halford as a side band)
Bruce Dickinson (Samson, Iron Maiden)

And,you might not remember this,but the late Dusty Springfield was part of a group with her brother Tim and another man,who did 1962's ORIGINAL version of "Silver Threads and Golden Needles",and ranked #20,(Dusty is clearly heard on female lead),and went on to a VERY successful solo career.And remember Dion Di Mucci?He was with the group "Dion and The Belmonts",and had a whole string of hits in the 1960's as a solo artist when the group broke up.Remember "Runaround Sue"(1961)?"Lonely Teenager"(1960)?"The Wanderer"(1961)?,"Lovers Who Wander"(1962),?"Love Came To Me"(1962)?Ruby Baby(1963),and "Abraham Martin and John (1968)?

Oops!I forgot:The group Dusty Springfield was in was called "The Springfields".

Cass Elliott (The Mamas and The Papas, The Mugwamps)

Fergie (The Black Eyed Peas)

Justin Timberlake ('N Sync)

Will Smith (DJ Jazzy Jeff and the Fresh Prince)

Tarja Turunen (Nightwish)

Rob Zombie (White Zombie)

No mention of Steve Winwood? (Traffic, Spencer Davis Group, Blind Faith)

And obviously...I'll mention two of my favorite bands with musicians who went solo....

Robin Trower (Procol Harum)
Steve Page (Barenaked Ladies)

Steve Vai (Frank Zappa band, Whitesnake)
Yngwie Malmsteen (Steeler, Alcatrazz)

And,the late "Papa John"John Phillips of "The Mamas and the Papas"(and Mackenzie Phillips'late father)had a solo career briefly with the 1970 #32 song "Mississippi".

King Diamond (Mercyful Fate)

No mention of Steve Winwood? (Traffic, Spencer Davis Group, Blind Faith)

Good one!

Ricky Martin (Menudo)

Smokey Robinson (The Miracles)
Beyonce (Destiny's Child)
Teddy Pendergrass (Harold Melvin and The Bluenotes)
David Ruffin (The Temptations)
Eddie Kendricks (The Temptations)
George Clinton (Parliament/Funkadelic)
Jeffrey Osborne (L.T.D.)
Babyface (The Deele, Mandrill)
Ronald Isley (The Isley Brothers)
Philip Bailey (Earth, Wind & Fire)
Larry Graham (Sly & the Family Stone, Graham Central Station)
Gerald Levert (Levert)
Chaka Khan (Rufus)
Patti LaBelle (LaBelle)
Charlie Wilson (Gap Band)
Morris Day (The Time)
Jody Watley (Shalamar)
Ann Nesby (Sounds of Blackness)
Bobby Brown (New Edition)
Johnny Gill (New Edition
Ralph Tresvant (New Edition
Bell Biv DeVoe (New Edition)
Sisqo (Dru Hill)
Lenny Williams (Tower of Power)
Anita Baker (Chapter 8)
El DeBarge (DeBarge)